Pulse detonation tool, method and system for formation fracturing

ABSTRACT

According to one aspect of the invention, a pulse detonation tool is provided for fracturing subterranean formations. The pulse detonation tool includes a pulse detonation combustor and creates an isolated zone within a wellbore. The tool generate a series of repeating supersonic shock waves that are directed into the subterranean formation to cause propagation of multiple fractures into the formation. According to another aspect of the invention, a method and system for fracturing a subterranean formation using pulse detonation is provided.

BACKGROUND

Formation fracturing is becoming an important tool for hydrocarbon production. There is an increasing need to produce hydrocarbons from wells in subterranean formations that contain a sufficient volume of hydrocarbon fluids, but have low permeability or restricted flow near the wellbore so that production is slow or difficult and, thus, not economical. In addition to oil and/or gas producing wells, formation fracturing is also an important tool for injection wells, storage wells, brine or water production wells, and disposal wells.

Flow of a fluid, such as oil or gas, through a porous medium, i.e. a subterranean formation, is directly related to the permeability of the formation. A formation with low permeability can occur naturally due to the geological conditions of the formation. Low permeability can also be caused by damage to the formation from drilling, cementing and perforating operations. Further, mature wells can incur damage from the buildup of fine particulates and contaminants. If permeability can be increased in these formations, more fluid can be recovered.

It is known that one way to increase production and permeability within a formation is artificial stimulation through “well fracturing.” Various fracturing procedures have been introduced and used, including: (1) hydraulic fracturing; (2) explosive fracturing; (3) various chemical treatments (usually acids); (4) high energy gas fracturing or controlled pulse fracturing; and (5) combinations of the above.

Originally, devices and materials such as nitroglycerin, dynamite or other high energy materials that produce “explosive” events were used to increase fluid flow around the wellbore of an oil or gas production well. This method of formation fracturing is called explosive fracturing and is associated with a rapid pressure rise over a short time period. The success of this method has been limited, however, due to safety hazards and because frequently the explosive compaction of the formation opposite to the explosion fracture causes formation and wellbore damage, which results in a decrease rather than increase of permeability.

Currently, the most common method of formation fracturing is hydraulic fracturing. Hydraulic fracturing increases formation permeability by slowly pumping a fluid into the formation, which in turn creates a slow pressure rise in the formation. Fluid pressure is steadily increased until the tensile strength of the rock formation is exceeded. At that point, a fracture will be initiated that propagates from opposite sides of the wellbore into the formation. Because the increased fluid pressure flows to the point of least resistance, a single bidirectional fracture typically is formed. Although this method is successful, the equipment and labor involved in hydraulic fracturing is extensive and expensive. There are also growing concerns regarding the environmental consequences associated with hydraulic fracturing due, in part, to the huge amounts of water that are required and the variety of chemicals that are used in connection with hydraulic fracturing.

The third type of well fracturing used in lieu of hydraulic fracturing or explosive fracturing is called “high energy gas fracturing” or “propellant fracturing.” This method employs propellant deflagration technology to create a more rapid pressure rise than that seen in hydraulic fracturing, but less rapid than during an explosive fracturing regime. “Deflagration” refers to the rapid burning of a material at faster rate than normal combustion, but at a rate slower than detonation. Propellant deflagration produces a good distribution of radial fractures around a wellbore and can be employed in lieu of hydraulic fracturing techniques as a more cost effective manner to create and propagate fractures. The resulting radial fractures, however, do not penetrate deep enough into the formation (i.e only 50-75 feet) and thus it is often necessary to combine them with, for example, hydraulic fracturing or chemical treatments.

Each of the current methods of formation fracturing thus have drawbacks. Pulse detonation devices, in general, are known and have been considered for use in jet aircrafts for propulsion, coal gasification, impulsive cleaning systems and medical cleaning devices. A pulse detonation device is an apparatus which produces high pressure exhaust from a series of repetitive detonations within a detonation chamber. The process is a constant volume heat addition process. The gaseous fuel is detonated within a chamber, causing a pulse detonation wave which propagates at supersonic speeds. The detonation wave compresses the fluid within the chamber, increasing its pressure, temperature and density, and producing a series of high-intensity, high-decibel blasts.

In pulse detonation combustors, a mixture of fuel and oxidizer, such as air, is ignited and either transitioned from deflagration to detonation, or detonated via direction initiation (DI), so as to produce detonation waves. The deflagration to detonation transition (DDT) or DI of detonation typically occurs in a tube or pipe structure.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect of the invention, a pulse detonation tool for fracturing subterranean formations is adapted to be lowered into a production tubing disposed within a wellbore and comprises a first and second sealing mechanism configured to create an isolated zone having an axis parallel to and extending through the production tubing, wherein the first sealing mechanism has at least one inlet port configured to allow a fuel and an oxidizer to flow into a pulse detonation combustor disposed within the isolated zone and wherein the first sealing mechanism is further configured to connect to an oxidizer and a fuel source by way of a fluid injection line extending from the surface through the tubing; at least one valve assembly to achieve controlled delivery of the fuel and oxidizer to the pulse detonation tool; the pulse detonation combustor comprising: a combustion region defining a fluid flow path; a mixing region for producing a flammable mixture comprising a controlled volume of fuel and oxidizer, wherein the mixing region is in fluid communication with the at least one inlet port and the combustion region; an ignition device configured to periodically ignite the flammable mixture; and means for initiating a series of repeating detonations that generate a series of repeating supersonic shock waves, wherein the shock waves are directed into the subterranean formation to cause propagation of multiple fractures into the formation.

Another aspect of the invention includes a method of fracturing subterranean formations comprising the steps of: (a) deploying at least one pulse detonation combustor into production tubing disposed within a wellbore; (b) positioning the pulse detonation combustor in an isolated zone within the wellbore, wherein the isolated zone is adjacent to a portion of the formation to be fractured; (c) commencing a pulse detonation cycle by, (i) creating a flammable mixture comprised of a fuel and oxidant mixture in the pulse detonation combustor by injecting a controlled amount of fuel from a fuel source and controlled amount of oxidant from an oxidant source into the pulse detonation combustor, wherein both the fuel source and oxidant source are located at the surface; (ii) igniting the fuel and oxidant mixture to cause a detonation, wherein the detonation within the pulse detonation combustor generates a supersonic shockwave; and (iii) purging combustion products of the detonation from the pulse detonation combustor; (d) directing the shockwave into the subterranean formation; and (e) repeating steps (i)-(iii) at a selected time and frequency sufficient to generate a series of repeating supersonic shock waves, thereby causing propagation of multiple fractures in the formation.

In another aspect of the invention, a method of fracturing subterranean formations comprises the steps of establishing a wellbore extending to the subterranean formation; and deploying a pulse detonation apparatus for generating repeating, supersonic shockwaves within the interior of the wellbore at a selected time and frequency sufficient to produce multiple fractures in the subterranean formation without causing damage to the wellbore and further extend the fractures until at least one hydrocarbon fluid fracture is intersected.

Another aspect of the invention includes a system for fracturing a subterranean/subterranean/geologic formation, comprising: means for establishing a wellbore extending to the subterranean formation; and a pulse detonation apparatus for generating repeated, supersonic pulses within the interior of the wellbore with a total number of pulses to produce multiple fractures in the subterranean formation and further extend the fractures until at least one hydrocarbon fluid fracture is intersected.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments of the invention which are schematically set forth in the figures. Like reference numerals represent corresponding parts.

FIG. 1 illustrates a cross-sectional view of a pulse detonation combustor according to an exemplary embodiment of the present invention;

FIG. 2 is a schematic illustration of a pulse detonation tool disposed within a wellbore according to an exemplary embodiment of the present invention as shown in FIG. 1;

FIG. 3 illustrates a cross-sectional view of a pulse detonation combustor according to yet another exemplary embodiment of the present invention;

FIG. 4 is a schematic illustration of a pulse detonation tool disposed within a wellbore according to the exemplary embodiment of the present invention as shown in FIG. 3;

FIG. 5 illustrates a cross-sectional view of a pulse detonation combustor according to yet another exemplary embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention will be explained in further detail by making reference to the accompanying drawings, which do not limit the scope of the invention in any way.

FIG. 1 depicts a cross-sectional side view of an illustrative pulse detonation tool 100, hereinafter referred to as “tool 100,” for formation fracturing according to various exemplary embodiments of the present invention. In FIG. 2, tool 100 is shown suspended within a wellbore 304 that should be understood as extending from ground level into a subterranean formation 305.

Turning now to FIG. 1 in which an embodiment of the present invention is depicted. In one or more embodiments, tool 100 contains at least one pulse detonation combustor 101. As used herein, a “pulse detonation combustor” PDC is understood to mean any device or system that produces both a pressure rise and velocity increase from a series of repeated detonations or quasi-detonations within the device. A “quasi-detonation” is a supersonic turbulent combustion process that produces a pressure rise and velocity increase higher than the pressure rise and velocity increase produced by a deflagration wave. As used herein, “detonation” includes both detonations and quasi-detonations.

Embodiments of PDCs include a means of igniting a fuel/oxidizer mixture, for example a fuel/air mixture, and a detonation chamber, in which pressure wave fronts initiated by the ignition process coalesce to produce a detonation wave. Each detonation or quasi-detonation is initiated either by external ignition, such as spark discharge or laser pulse, or by gas dynamic processes, such as shock focusing, autoignition or by another detonation (i.e. cross-fire). Pulse detonation may be accomplished in a number of types of chambers including detonation tubes, shock tubes, resonating detonation cavities and annular detonation chambers, for example. In addition, a PDC can include one or more detonation chambers. The structure and construction of the pulse detonation combustor 101 is that of any known pulse detonation combustor type device, and the present invention is not limited in this regard.

In one or more embodiments, pulse detonation combustor 101 includes an ignition chamber 110 and a combustion region, or detonation chamber, 120. The ignition chamber 110 and the detonation chamber 120 can be formed as a contiguous region, as in FIG. 1, or discrete chambers (not illustrated).

In the embodiment shown in FIGS. 1-2, pulse detonation combustor 101 includes a tubular housing 102, an ignition device 10 disposed within ignition chamber 110, detonation chamber 120 formed within tubular housing 102, a fuel/oxidizer mixing region or chamber 130, at least one fuel inlet port 50 that supplies oxidizer and fuel to pulse detonator combustor 101, and at least one valve assembly 80. The diameter of tubular housing 102 can be varied to fit and preferably generally fills up the diameter of production tubing 300 or wellbore 304.

As used herein, “wellbore”, “borehole” and “well” are used interchangeably and should not be considered of varying scope.

Tubular housing 102 is preferably fabricated from a steel or like material having sufficient wall thickness and strength that it will not be destroyed during the pulse detonation cycle. Similarly, each of the components of tool 100 described herein should be formed from strong, durable materials and securely mounted so as to withstand the high pressures, repeated shock waves and thermal deformations from the repeated detonations and to resist fluid pressure in the wellbore during the pulse detonation cycle.

In the embodiment shown in FIGS. 1-2, tubular housing 102 has a first end 103 and a second end 104 and a circular side wall 105. For example, and as shown in FIG. 2, tubular housing 102 is an elongated, cylindrical shock tube closed at both ends (103/104). Circular side wall 105 of tubular housing 102 has an outside diameter smaller than the inside diameter of the production tubing 300, and is adapted to be lowered into and subsequently removed from a wellbore 304. In the embodiment shown here, tubular housing 102 also has discharge port 70 for venting combustion products or excess fuel or oxidant. One of ordinary skill in the art would recognize that port 70 could be eliminated or modified if the fuel/oxidizer mixture results in combustion products that condense into water or can readily dissolve into the water within the wellbore , or that other means for venting known in the art could be used to vent combustion products back to the surface or downhole.

Configured as a shock tube, tubular housing 102 contains and isolates the flammable mixture from production tubing 300 so as to confine combustion, or detonation, waves 20 generated by the repeating detonations to within tubular housing 102. Tubular housing 102 further comprises a plurality of spaced nozzles 80 along the length of the circular side wall 105. Nozzles 80 are located at spaced intervals. Each nozzle 80 extends radially outward and is configured to direct or aim the shock waves 30 generated by the repeating detonations in a substantially axial direction into the subterranean formation 305. Nozzles 80 are adapted so that tubular housing 102 can be lowered into and subsequently removed from wellbore 304. For example, in one embodiment, nozzles 80 can be telescoping.

In the embodiment shown here, fuel inlet port 50 is also configured to supply a buffer to pulse detonator combustor 101, wherein the buffer is any nonflammable fluid. In this embodiment, the buffer is different from the fuel and oxidant and introduced separately. One of ordinary skill in the art would recognize, however, that a buffer and buffer inlet is not required if there is sufficient time between detonation cycles to allow the combustion products remaining in the pulse detonation combustor 101 to cool below autoignition temperature of the fuel-oxidizer mixture for the next detonation cycle. Furthermore, even if a buffer is required, one of ordinary skill in the art would recognize that either the fuel or oxidizer could be used as the buffer by configuring and adjusting valve assembly 80, thereby eliminating the need for a separate buffer and buffer inlet.

Referring now to FIG. 2, tool 100 also includes a first sealing mechanism 200 and a second sealing mechanism 201. Sealing mechanisms 200/201 are configured to create an isolated zone 210 having an axis parallel to and extending through wellbore 304. Sealing mechanisms 200/201 comprise any suitable means for sealing, such as an expandable packer shown in FIG. 2 or tubing (not illustrated). First sealing mechanism 200 is located uphole of tubular housing 102 and second sealing mechanism 201 is located downhole of tubular housing 102. First and second sealing mechanisms 200/201 are positioned to align isolated zone 210 with perforations 306 in the production tubing 300, well casing (not shown), or both.

First sealing mechanism 200 has at least one first inlet port 50 configured to allow a fuel, an oxidizer and a buffer to flow into pulse detonation combustor 101 disposed within isolated zone 210. In the embodiment shown in FIG. 2, the at least one first inlet port 50 comprises fuel inlet port 50 a, oxidizer inlet port 50 b, and buffer inlet port 50 c. First inlet ports 50 a-c are configured in accordance with technology known to a skilled artisan in pulse detonation technology. Furthermore, one of ordinary skill in the art could configure a single inlet port 50 to achieve flow communication with the fuel source 302, oxidizer source 301 and/or buffer source 303 (not shown). Although not shown in FIGS. 1-2, in certain embodiments, oxidizer source 301 is the same as buffer source 303. For example, in practicing the invention, one skilled in the art could utilize air as both the oxidant and the buffer. Accordingly, in certain embodiments oxidizer inlet 50 b and buffer inlet 50 c are one and the same.

Additionally, in the embodiment shown in FIGS. 1-2, tubular housing 102 has at least one second inlet, or injection, port 51 configured to allow a fuel, an oxidizer and a buffer to flow into combustor 101. As shown in FIG. 1, the at least one second inlet port 51 comprises multiple inlet ports, more specifically fuel inlet port 51 a, oxidizer inlet port 51 b, and buffer inlet port 51 c. Second inlet ports 51 a-c are configured in accordance with technology known to a skilled artisan in pulse detonation technology. For example, in other embodiments, fuel is introduced co-axially through fuel inlet port 50 a (see FIG. 3), or oxidizer is supplied to combustor 101 downstream of the fuel injection via inlets 50 b (not shown). As used herein, “downstream” refers to a direction of flow of at least one of fuel, or oxidizer or buffer. Accordingly, the inlet ports, or injectors, can be arranged in various other locations such as perpendicular to the flow, at a tangential angle to the flow (to induce swirl), or at an angle in conjunction with a suitably shaped wall to help promote mixing. Any known mechanism for injection can be used such as air-blast atomization, pressure-atomization, etc. By way of further example, one of ordinary skill in the art could configure a single inlet port to achieve injection of the fuel, oxidizer and/or buffer into combustor 101. Additionally, as discussed above, in certain embodiments wherein oxidizer source 301 is the same as buffer source 303, oxidizer inlet 51 b and buffer inlet 51 c are the same inlet port.

Referring again to FIG. 2, tool 100 is further configured to connect to an oxidizer source 301, fuel source 302 and buffer source 303 by way of multiple fluid injection lines 307, 308, 309 extending from the surface through the wellbore 304 or production tubing 300. In practicing the invention, fuel source 302 can be a liquid fuel or a gaseous fuel. Furthermore, tool 100 is operable with a plurality of different fuels including, but not limited to, gaseous fuels, such as, hydrogen, ethylene, natural gas, or propane, liquid fuels, such as, gasoline, kerosene, or aviation fuels, and a plurality of oxidizers including, but not limited to, air. Examples of suitable fuel/oxidizer combinations, or flammable mixtures, would include hydrogen/oxygen or methane/oxygen, both of which would be delivered in gaseous form only. Another example of a suitable flammable mixture would be propane/oxygen, wherein the propane would be in liquid form. Liquid rocket propellants using nitrous oxide or liquid oxygen as an oxidizer can also be used. Likewise, liquid explosives could be used as a fuel source, such as nitromethane mixed with dietylenetriamine or ethylenediamine. It should be noted, however, that the specific fuel/oxidizer combination selected will be dictated by operational parameters and characteristics within the wellbore 304 and subterranean formation 305, as well as the desired characteristics of fractures 400 within the formation, in order to optimize performance and limit damage within the formation or wellbore. The invention is not limited to the use of the above-identified fuel/oxidizer combinations and any suitable flammable mixture known to those skilled in the art of pulse detonation technology can be used in order to achieve the desired shock waves and resulting fracture geometry within the formation.

Tool 100 also comprises at least one valve assembly disposed within tubular housing 102. The at least one valve assembly is configured to achieve controlled delivery of the fuel, oxidizer and buffer to tool 100. In the embodiment shown in FIG. 1, the at least one valve assembly comprises fuel valve 80 a, oxidizer valve 80 b, and buffer valve 80 c.

As used herein, the term “valve” or “valve assembly” is intended to describe any device that turns on and off a flow at a high frequency, namely, faster than or equal to the time scale of one pulse detonation combustion cycle. Valves 80 a-c are configured in accordance with pulse detonation technology known to one of ordinary skill in the art, and may be either a passive check valve or active valve, or a combination of both. Furthermore, one of ordinary skill in the art could configure a single valve to achieve controlled delivery of the fuel, oxidizer and/or buffer. As shown in FIG. 1, fuel valve 80 a and oxidizer valve 51 b are positioned within tubular housing 102 downstream of fuel inlet port 51 a and oxidizer port 51 b, and both are configured to only allow fuel and oxidizer to flow into the mixing chamber 130 periodically.

Although not shown, in other embodiments, the valve assembly is located outside of tubular housing 102 but within isolated zone 210. In still further embodiments, the valve assembly is located upstream of the isolated zone 210 defined by sealing mechanisms 200/201. For example, in these embodiments, fuel valve 51 a is disposed between the fuel source and inlet port 51 a, and oxidizer valve 51 b is disposed between the oxidizer source and inlet port 51 b. In exemplary embodiments, tool 100 also comprises means for preventing backflow of the fuel or flammable mixture towards the surface within the production tubing 300.

Pulse detonation combustor 101 is detonated by suitable detonation means connected to ignition device 10. In this embodiment, ignition device 10 is located within ignition chamber 110 and is arranged downstream from the fuel and oxidizer inlets. Ignition device 10 comprises at least one ignition point, as shown in FIG. 1, or may comprise multiple ignition points (as shown in FIG. 3). Ignition device 10 can be, but is not limited to being, a spark plug, a plasma igniter, and/or a laser source, or any suitable device. Ignition device 10 is configured to periodically ignite the flammable mixture and, as such, is fabricated from suitable materials to allow device 10 to achieve multiple ignition events without being destroyed or consumed during the repeating detonation cycles.

In further embodiments, ignition device 10 comprises an external power source, timing device and remote signaler configured to remotely ignite the flammable mixture exiting mixing chamber 130, wherein the timing of ignition is predetermined and controlled via one of ordinary by one skill in the art. In an embodiment where there is no separate mixing chamber, the location of the ignition device 10 is arranged based upon the optimum ignition location for fuel-oxidizer mixing. For example, the ignition device 10 can be placed downstream of the fuel inlet port to provide time for the fuel to mix with the oxidizer. Ignition device 10 should also be placed upstream of any detonation-creating obstacles that may be located in the detonation chamber.

Ignition chamber 110 and detonation chamber, or combustion region, 120 define a fluid flow path in flow communication with mixing chamber 130, as illustrated in FIG. 1. Mixing chamber 130 is also in fluid communication with the at least one inlet port and the detonation chamber 120. Mixing chamber 130 is configured to produce a flammable mixture comprising a controlled volume of fuel and oxidizer and to promote uniform flow into the ignition chamber 110. For example, the geometry of mixing chamber 130 can be a cylindrical chamber wherein the fuel and oxidizer mix as a result of turbulence created by the fuel and oxidant jet inlet interaction. Mixing chamber 130 can also comprise a perforated plate or a geometry to induce swirl or other turbulence for example. In practicing the invention, any suitable flow mixing element can be used to promote the uniform flow of the fuel/oxidant mixture into the ignition chamber 110 and additional geometry known to those skilled in the art can function to enhance mixing and uniform flow if desired.

Pulse detonator combustor 101 also comprises means for initiating a series of repeating detonations that in turn generate a series of repeating supersonic, high impulse shock waves 30. In one embodiment, as shown in FIG. 1, the means for initiating a series of repeating detonations comprise a series of obstacles disposed along the fluid flow path within detonation chamber 120. More specifically, in the embodiments shown herein, detonation chamber 120 includes an obstacle field or center body 90 to promote turbulence within the detonation chamber 120. The center body 90 is often referred to as deflagration to detonation transition (DDT) geometry. DDT geometry enhances the deflagration to detonation transition process by increasing turbulence in the detonation chamber 120. There are a variety of DDT geometries. The overall length and diameter of the center body 90 will be dictated by operational parameters and characteristics within the wellbore 304 and subterranean formation 305, as well as the desired characteristics of fractures 400 within the formation, in order to optimize performance.

It is to be noted that the invention is not limited to the use of a particular DDT geometry. Any suitable DDT geometry can be used to increase turbulence. Similarly, detonation chamber 120 can be arranged without DDT geometry or with other means for initiating a series of repeating supersonic shock waves known to those skilled in the art. For example, in alternate embodiments, the means for initiating a series of repeating detonations comprises direct initiation (DI) detonation methods, such as a high energy laser, spark, or other shock-to-detonation methods suitable for achieving direction initiation (DI) of detonation. In practicing the invention, one of ordinary skill in the art would recognize that DI of detonation is preferable over DDT if circumstances allow for lower frequency requirements, high pressure in the well bore, and the appropriate fuel-oxidizer mixtures. In addition, the fuel-oxidizer ratio can be supplied so that there is a slightly fuel-rich mixture in the ignition chamber 110 to improve the selected means for initiating a series of repeating detonations . This can be accomplished by controlling the flow of fuel and oxidizer into the ignition chamber 110 via valve assembly 80.

Referring to FIG. 2, the resulting shock waves 30 are directed into the subterranean formation 305 and create propagation of multiple fractures 400 (not shown) into formation 305 without causing damage to the surrounding wellbore 304. In the embodiment shown here, combustion or detonation waves 20 are confined to tubular housing 102. In practicing the invention, the resulting shock waves 30 are in the range of about 10 to 16-times the initial pressure in the formation. For example, in one embodiment with an initial pressure of 100,000 psi-160,000 psi. If the initial pressure in the wellbore is lower, the shock waves could be approximately in the range of about 10,000 psi to 24,000 psi, and preferably 15,000 psi. The frequency of operation of the particular embodiment of pulse detonation combustor 101 shown in FIGS. 1-2 is in the range from about 0.1 millihertz to 100 Hertz. It should be noted, however, that the firing frequency, in part, is determined by the time required to purge and refill the volume within pulse detonation combustor 101 with the fuel/oxidant flammable mixture. As such, in certain embodiments (see, for example, see FIGS. 3-5), the firing frequency will be in the range of about 0.1 millihertz to 5 Hz, preferably 0.01-0.2 Hz. After the requisite number of firing cycles, multiple fractures 400 will extend into formation 305 at a distance in the range of about 50 meters to about 300 meters, and in certain embodiments a distance of about 100 meters.

In practice, and referring again to FIG. 2, to fracture a subterranean formation and recover hydrocarbons, a wellbore 304 is drilled vertically and/or horizontally into subterranean formation 305 to some depth below the surface. The wellbore 304 can be lined with production tubing or casing, to strengthen the walls of the well, or not. For example, individual lengths of metal tubing can be secured together to form a casing string which is positioned within a well bore, thus increasing the integrity of the wellbore and providing a path for hydrocarbons to flow to the surface. If casing is used, and to further strengthen the walls of the wellbore 304, the annular area formed between the casing and the borehole 304 can be filled with cement to permanently set the casing in the wellbore. The casing can then be perforated at a location where fracturing is to take place using a perforation tool that is lowered into the wellbore from the surface.

Tool 100, which includes sealing mechanism 200/201, is then lowered into the wellbore 304 to a depth adjacent to the particular section of subterranean formation 305 to be fractured. In the example shown in FIG. 2, tool 100 is positioned adjacent to perforations 306. Tool 100 can be lowered and positioned into a wellbore 304 by conventional means, such as on a wireline, slickline, production tubing, pipe tubing, coiled tubing, or any combination thereof, or any other technique known or yet to be discovered by a skilled artisan.

Prior to lowering the tool 100 into the wellbore 304, the location, distance and direction of the at least one natural hydrocarbonaceous fluid fracture is determined. Furthermore, the characteristics of the subterranean formation 305 are determined and used to construct a pulse detonation combustor 101 with the desired pulse detonation characteristics for the particular formation such that optimum radial fractures can be achieved within the subterranean formation 305. These determinations can be made by geologists and others skilled in the art.

Once tool 100 is properly positioned within the well to create an isolated zone 210 adjacent to both the perforations 306 and the portion of the formation to be fractured, the pulse detonation cycle is commenced. A controlled amount of fuel from fuel source 302 located on the surface, and a controlled amount of oxidizer from oxidizer source 303 located on the surface, are injected into mixing chamber 130 via fuel supply line 308, oxidizer supply line 309, and the respective inlet ports, to create a flammable mixture comprised of a predetermined fuel and oxidant mixture in the pulse detonation combustor 101.

The flammable mixture flows from the mixing chamber 130 to the ignition chamber 110 and is ignited. The flame then propagates into the detonation chamber and detonates within the detonation chamber 120, which in turn generates a supersonic shockwave. In the embodiment shown in FIGS. 1-2, the shockwave is then directed radially into the subterranean formation via nozzles 80 that are aligned with perforations 306. Following the first detonation cycle, the combustion products of the detonation are then purged from detonation chamber 120 and, if necessary, a buffer is introduced into the pulse detonation combustor 101 before commencing the next pulse detonation cycle.

As illustrated in FIG. 2, the generated shock waves 30 resulting from pulse detonation are at supersonic velocities (for example, approximately 2000 m/s), and high pressure in the range of about 10 to 16 times the initial pressure of the formation. The shock waves 30 are directed radially into the formation through nozzles 80 formed in tubular housing 102. The generated shock waves 30 pass through perforations 306 formed in the well casing to open and extend existing fractures. The shock waves also may clear out blockages of materials that have been deposited in existing fractures.

These same steps are repeated at a predetermined, calculated time and frequency sufficient to generate a series of repeating, high impulse supersonic shock waves that cause propagation of multiple fractures 400 to extend further and further into the formation with each cycle. In practice, the precise number of repeated detonation “pulses”, or cycles, required to reach the desired fracture length will depend on the nature and characteristics of the formation. More specifically, in one example rock formation, the propagation of a fracture 1 cm in length requires 1000 detonation cycles. Therefore, in order to extend the fracture out 100 m into the formation, the pulse detonation cycle will require 10,000,000 pulses or cycles.

The repeating, high impulse shock waves have a pressure above the maximum fracture extension pressure but below that which would cause casing, wellbore or formation damage. In other words, the shock waves create a pressure loading rate sufficient to fracture the rock and create multiple radial fractures at the wellbore and extending into the formation, but low enough to avoid crushing the formation or casing adjacent to the wellbore. As such, the repeated “pulses” will allow fractures to continue their extension into the formation to obtain the required distances to reach the hydrocarbons in the formation.

In another embodiment, shown in FIGS. 3-4, circular side wall 105 of tubular housing 102 is formed by the production tubing 300, and the first and second end of the tubular housing (103/104) are formed by the first sealing mechanism 200 and second sealing mechanism 201, located uphole and downhole respectively. As further shown in FIG. 3, concentric liner 115 is positioned within tubular housing 102 with an outside wall that has a diameter smaller than the diameter of production tubing 300, and wherein the uphole and downhole seal 200/201 are adapted to be mounted to each end of the cylindrical tube to form an annular combustion region 120 between the outside wall of concentric liner 115 and production tubing 300. In this embodiment, mixing region 130 is disposed in fluid communication with the at least one inlet port 50 and annular combustion region 120. Furthermore, annular combustion region 120 is aligned with the perforations 306 to provide direct flow of the flammable mixture into the fractures of the subterranean formation 305. Pulse detonation combustor 101, according to this exemplary embodiment, includes multiple ignition sources 10.

In this embodiment, both the combustion waves 20 and shock waves 30 generated by the repeating detonations extend into the fractures 400. More specifically, the flammable mixture is pushed into perforations 306 and thus into the subterranean formation. In one embodiment (not shown), combustion can be initiated in the production tubing and then transition into the fueled perforations or fractures within the formation. Alternatively, and as shown in FIG. 3, multiple ignition devices 10 can be positioned so that combustion is initiated directly into the formation. With this embodiment, both the detonation waves and shock waves will occur directly in the rock formation, resulting in a higher pressure loading.

FIG. 5 illustrates another exemplary embodiment of tool 100. In this embodiment, circular side wall 105 of tubular housing 102 is once again formed by the production tubing 300, and the first and second end of the tubular housing (103/104) are formed by the first sealing mechanism 200 and second sealing mechanism 201, located uphole and downhole respectively.

As shown in FIG. 5, isolated zone 210 within the production tubing forms the detonation, or combustion, region 120 of pulse detonation combustor 101 and the entire volume between the first and second sealing mechanisms 200/201 will be filled with the flammable mixture. In contrast to the embodiment shown in FIGS. 3-4, because the flammable mixture is contained within production tubing 300, combustion waves 20 will be confined to production tubing 300 and will not extend into the formation. Shock waves 30 extend radially into the formation. The arrangement of the ignition device 10 and fuel and oxidizer inlet ports and valve assembly are similar to those in the previous embodiments, and a mixing region is provided to promote uniform flow. Even more significantly than the other embodiments, the firing frequency will largely be determined by the time required to purge and refill the volume within tubular housing.

As used herein, the terms “tubular housing” and “concentric liner” include tubes having a circular or alternatively non-circular cross-section. Exemplary hollow carriers or concentric liners include cylindrical tubes and tubes having polygonal cross-sections, such as, for example, hexagonal tubes. Each of the circular and non-circular cross-sections identified above may have a continuous cross sectional area, or the cross sectional area may vary. For example, in the exemplary embodiments shown in FIGS. 1-2, the cross-section of the ignition chamber 110 can be larger than the cross section of the detonation chamber 120. Likewise, the volume of the detonation chamber 120 can be larger than that of the ignition chamber 110, wherein the specific ratio is set to optimize the performance based upon the characteristics of the subterranean formation or wellbore. Cross-sectional area variations in the ignition chamber 110 and the detonation chamber 120 allow for control of the bulk flow velocity, which in turn enhances fuel-air mixing, initial flame growth, DDT turbulence, minimizes loads on the upstream components in tool 100, and can reduce the requisite length of pulse detonator combustor 101, for example. By reducing the length of the combustor, the run-up time for DDT will be reduced, which will then enable combustor 101 to operate at higher frequencies. Higher frequency will generate more pressure rise and increase the usable output of tool 100 in the subterranean formation.

In further embodiments, tool 100 comprises means to introduce a proppant into the fractures. Fractures have a tendency to close or collapse once the pressure in the formation is relieved. To prevent such closing when the fracturing pressure is relieved, fracturing techniques often employ a granular or particulate material, referred to as a “proppant,” that is left behind in the fractures. The proppant is used to keep the fracture open and thus provide a flow path through which hydrocarbons can flow. In one or more of the embodiments disclosed herein, the means to introduce proppants include mixing the proppant with the fuel, the oxidizer or both and the introduction of the proppant would start with the deflagration event pushing the proppant into the formation. In other embodiments, proppants could be entrained with the fracturing fluid or introduced via other means as is known in the art. A variety of proppants can be used depending on the geological conditions of the formation, including particulate materials, such as sand, glass beads and ceramic pellets, which create a porous structure.

It is noted that the above embodiments have been shown with respect to a single pulse detonation combustor. However, the concept of the present invention is not limited to single pulse detonation combustor. Furthermore, it should be expressly understood that any desired number of pulse detonation combustors could be employed in series, and that the dimensions, configurations, and compositions of tool 100 is within the discretion of the skilled artisan to meet the needs of a particular well.

Finally, in exemplary embodiments, a computer program can be used to model the pulse detonation cycle to predict the resulting generation of fracture propagation, and thereby determine a suitable configuration of tool 100 for fracture propagation in the surrounding formation. Tool 100 may also be equipped with remote pressure and temperature sensors, transducers, acoustic sensors, accelerometers (vibration sensors), chemical sensors such as oxygen sensor to confirm presence or absence of oxidizer] or other means known in the art to accurately monitor the pulse denotation cycle within the wellbore.

Although the apparatus and method of the invention is disclosed with examples that incorporate a cased well with production tubing that has been perforated, the apparatus and method of the invention are equally applicable to an open hold completion of a well.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

What is claimed is:
 1. A pulse detonation tool for fracturing subterranean formations, adapted to be lowered into production tubing within a wellbore, and comprising: a first sealing mechanism and a second sealing mechanism configured to create an isolated zone having an axis parallel to and extending through the production tubing, wherein the first sealing mechanism has at least one inlet port configured to allow a fuel and an oxidizer to flow into a pulse detonation combustor disposed within the isolated zone and wherein the first sealing mechanism is further configured to connect to an oxidizer and fuel source by way of a fluid injection line extending from the surface through the tubing; at least one valve assembly to achieve controlled delivery of the fuel and oxidizer to the pulse detonation combustor, the pulse detonation combustor comprising: a combustion region defining a fluid flow path; a mixing region for producing a flammable mixture comprising a controlled volume of fuel and oxidizer, wherein the mixing region is in fluid communication with the at least one inlet port and the combustion region; an ignition device configured to periodically ignite the flammable mixture; and means for initiating a series of repeating detonations that generate a series of repeating supersonic shock waves, wherein the shock waves are directed into the subterranean formation to cause propagation of multiple fractures into the formation.
 2. The apparatus of claim 1 wherein the first sealing mechanism and second sealing mechanism are positioned to align the isolated zone with perforations in the production tubing, a well casing, or both.
 3. The apparatus of claim 1 wherein the shock waves are about 10 to about 16 times the initial pressure in the wellbore.
 4. The apparatus of claim 3 wherein the shock waves are in the range of about 10,000 psi to 24,000 psi, and preferably 15,000 psi.
 5. The apparatus of claim 1 wherein the frequency of operation of the pulse detonation combustor is at a frequency in the range from about 0.1 millihertz to 100 hertz, preferably 0.2 millihertz to 5 hertz, and more preferably 0.01-0.2 Hertz.
 6. The apparatus of claim 1 wherein the means for initiating a series of repeating detonations comprise a series of obstacles disposed along the fluid flow path of the combustion region.
 7. The apparatus of claim 1 wherein the at least one inlet port comprises a fuel inlet port and an oxidizer inlet port.
 8. The apparatus of claim 1 wherein the at least one inlet port and the at least one valve assembly are further configured to introduce a buffer into the pulse detonation combustor after each detonation to purge the combustor of combustion products.
 9. The apparatus of claim 8 wherein the buffer is non-flammable and selected from the group consisting of the oxidant, the fuel or air .
 10. The apparatus of claim 8 wherein the at least one inlet port further comprises a buffer inlet port.
 11. The apparatus of claim 1 wherein the at least one valve assembly comprises a plurality of valves and valve-actuating means.
 12. The apparatus of claim 1 wherein the at least one valve assembly comprises a fuel valve disposed between the fuel source and inlet port, the fuel valve configured to only allow fuel to flow into the combustion chamber periodically.
 13. The apparatus of claim 1 further comprising means for preventing backflow of the fuel or flammable mixture towards the surface within the tubing.
 14. The apparatus of claim 1 wherein the pulse detonation combustor comprises a tubular housing having a first end and a second end and a circular side wall.
 15. The apparatus of claim 14 wherein the tubular housing is formed by an elongated, cylindrical shock tube closed at both ends and disposed within the production tubing, wherein the shock tube has an outside diameter smaller than the inside diameter of the production tubing and is adapted to be lowered into and subsequently removed from the wellbore.
 16. The apparatus of claim 15 wherein the shock tube contains and isolates the flammable mixture from the production tubing surrounding the shock tube so as to confine combustion waves generated by the repeating detonations to within the shock tube.
 17. The apparatus of claim 15 wherein the shock tube further comprises a plurality of spaced nozzles along the length of the circular side wall, each nozzle extending radially outward and configured to aim the shock waves in a substantially axial direction into the formation.
 18. The apparatus of claim 17 wherein the nozzles are adapted so that the shock tube can be lowered into and subsequently removed from the wellbore.
 19. The apparatus of claim 14 wherein the circular side wall of the tubular housing is formed by the production tubing.
 20. The apparatus of claim 19 wherein the first and second end of the tubular housing are formed by the first and second sealing mechanisms, respectively.
 21. The apparatus of claim 20 wherein the flammable mixture is contained within the production tubing so as to confine combustion waves generated by the repeating detonations to the production tubing.
 22. The apparatus of claim 20 further comprising a cylindrical tube with an outside wall, wherein the outside wall has a diameter smaller than the diameter of the production tubing, and wherein the first and second sealing mechanisms are adapted to be mounted to each end of the cylindrical tube to form an annular combustion region between the outside wall of the cylindrical tube and the production tubing.
 23. The apparatus of claim 22 wherein the mixing region is disposed in fluid communication with the at least one inlet port and the annular combustion region, and wherein the annular combustion region is aligned with the perforations in the well casing to provide direct flow of the flammable mixture into the fractures of the subterranean formation.
 24. The apparatus of claim 23 wherein the combustion wave generated by the repeating detonations extends into the fractures of the formation.
 25. The apparatus of claim 1 wherein the structural components of the pulse detonation combustor are made of materials sufficient to withstand repeated shock waves and thermal deformations from the repeated detonations and to resist fluid pressure in the wellbore.
 26. The apparatus of claim 1 wherein the ignition device comprises at least one ignition point.
 27. The apparatus of claim 1 wherein the ignition device comprises multiple ignition points.
 28. The apparatus of claim 1 wherein the ignition device comprises electrical ignition means or chemical ignition means.
 29. The apparatus of claim 1 wherein the ignition device comprises a remote signaler to remotely ignite the flammable mixture.
 30. The apparatus of claim 1 wherein the fuel is a liquid fuel or a gaseous fuel.
 31. The apparatus of claim 1 adapted to be lowered into a wellbore on a wireline, production tubing, coiled tubing, or any combination thereof.
 32. The apparatus of claim 1 wherein the at least one inlet port is configured to provide a continuous supply of air to the pulse detonation combustor during the operation of the combustor.
 33. The apparatus of claim 1 wherein the first and second sealing mechanisms comprise a pair of expandable packers.
 34. The apparatus of claim 1 further comprising means to introduce a proppant into the fractures.
 35. A method of fracturing subterranean formations comprising the steps of: (a) deploying at least one pulse detonation combustor into production tubing disposed within a wellbore; (b) positioning the pulse detonation combustor in an isolated zone within the wellbore, wherein the isolated zone is adjacent to a portion of the formation to be fractured; (c) commencing a pulse detonation cycle by, (i) creating a flammable mixture comprised of a fuel and oxidant mixture in the pulse detonation combustor by injecting a controlled amount of fuel from a fuel source and controlled amount of oxidant from an oxidant source into the pulse detonation combustor, wherein both the fuel source and oxidant source are located at the surface; (ii) igniting the fuel and oxidant mixture to cause a detonation, wherein the detonation within the pulse detonation combustor generates a supersonic shockwave; and (iii) purging combustion products of the detonation from the pulse detonation combustor; (d) directing the shockwave into the subterranean formation; and (e) repeating steps (i)-(iii) at a selected time and frequency sufficient to generate a series of repeating supersonic shock waves, thereby causing propagation of multiple fractures in the formation.
 36. The method according to claim 35 wherein the isolated zone is formed by a first and second sealing mechanism secured within the wellbore.
 37. The method according to claim 35 wherein the isolated zone is aligned with perforations in the production tubing, well casing, or both.
 38. The method according to claim 35 wherein the pulse detonation combustor comprises: a combustion region defining a fluid flow path; a mixing region for producing the flammable mixture comprising a controlled volume of fuel and oxidizer, wherein the mixing region is in fluid communication with the at least one inlet port and the combustion region; an ignition device configured to periodically ignite the flammable mixture; and means for initiating a series of repeating detonations.
 39. The method of claim 35 wherein the step of purging the pulse detonation combustor comprises introducing a buffer into the pulse detonation combustor after igniting a fuel and oxidant mixture from a previous cycle and before commencing a next pulse detonation cycle.
 40. The method of claim 35 further comprising the step of introducing a proppant into the fractures.
 41. The method according to claim 35 further comprising the step of controlling the time and frequency of the pulse detonation cycle by a programmable digital signal processor.
 42. The method according to claim 35 wherein the pulse detonation combustor provides shockwaves at a frequency in the range from about 0.1 millihertz to 100 hertz, preferably 0.2 millihertz to 5 hertz, and more preferably 0.01-0.2 Hertz.
 43. The method according to claim 35 wherein two or more pulse detonation combustors are connected in series.
 44. A method of fracturing subterranean formations comprising the steps of: establishing a wellbore extending to the subterranean formation; and deploying a pulse detonation apparatus for generating repeating, supersonic shockwaves within the interior of the wellbore at a selected time and frequency sufficient to produce multiple fractures in the subterranean formation without causing damage to the wellbore and to further extend the fractures until at least one hydrocarbon fluid fracture is intersected.
 45. A system for fracturing a subterranean/subterranean/geologic formation, comprising: means for establishing a wellbore extending to the subterranean formation; and a pulse detonation apparatus for generating repeating, supersonic pulses within the interior of the wellbore with a total number of pulses sufficient to produce multiple fractures in the subterranean formation and further extend the fractures until at least one hydrocarbon fluid fracture is intersected. 